Process for producing butterfly twin barium titanate single crystals and barium titanate mixture used therein



United States Patent Ofice 3,409,412 Patented Nov. 5, 1968 3409 412 PROCESS FOR PRODUCING BUTTERFLY TWIN BARIUM TITANATE SINGLE CRYSTALS AND BARIUM TITANATE MIXTURE USED THEREIN cutoif in the distribution in the large-particle region. The Nielsen method comprises a strict control of particle size distribution of starting barium titanate powder and an additional procedure of grinding which is liable to con- Hiromu Sasaki, Osaka-shi, Osaka-in, Japan, assignor to 5 taminate the starting material. Minor impurity is known Matsushita Electric Industrial Co., Ltd., Osaka, Japan t decrease the yield. g g qg gga gr g g ig k f ig g' i g iggg It is an object of this invention to provide an im- 40/40 proved process for the preparatlon o f sizable butterfly 4 Claims (CL 23 300) twin crystals of barium tltanate in a high y1eld based on the Remeika method, employing no particle size control. Th1s 1nve1 1t1on relates t a Improved method r g It is another object of the invention to eliminate addimg banum titanate single crystals having a so-called butterti n l ri din rocedure which may contaminate the fly wings-shaped habit and more particularly to composistarting powders. Hons 9 bar 1um maflate Powders as a Starting {natfirial- It has been discovered according to the present invenria l gl j gc fi i s g it iiugrl l v zfil l i zgl fj matetion that the molar ratio of barium oxide to titanium didevice utilizi: it 1 s h t i ahmemory oxide in the starting bar1um titanate powder has a subg S E g ys eresls 00p 6 aracter' stantial effect on the yield of butterfly twins. The essential 1st1c. (Electrical Englneerlng, volume 71, pages 916-922, f

. eatures of the present inventive method are based on the November 1952, I.R.E.Transact1ons on Component Parts, h th b t fl M h volume p 3 pages 341 March 1956 lscovery t at e u ter twln yle s ows a maxlrnum It is also Well known that barium titanate is crystallized villus: when the molar mu? of Panum oxlde mamum dioxide in the startlng barium titanate powder is at 1.20 in the form of either th1n plate or chunky cube dunng a t 1 r d1 f l slow cooling of its molten solution with a flux such as o Piglfiss-o 1 813ar Size" b potassium fluoride. The thin plate crystals have a right- Stole iomemc bar-mm tltanate- 1s rep-resfintfid y an 18 d man mar h e th h t f l 25 unity of said molar ratlo. The startmg bar1um t1tanate g ap e We 0 ates powder with said molar ratio higher or lower than umty mg Jomed Y i the angle approxlmately 40 is prepared by firing a mixture of barium carbonate and degrees ThlS pa r looks like butterfly wings; hence it is titanium dioxide in a desired molar ratio at 1200 C. to Ca led butterfly twin crystal. 1450 C. for one hour in air. Various compositions are The preparation method for the barium titanate twin t forth i Tabl I. An X-ray powder identification incrystals was reported by J. P. Remeika in the Journal of dicates that compositions with the molar ratio higher or the American Chemical Society, vol. 76, pages 940-941, lower than unity exist in two phases of barium titanate 1954. The first step in this procedure consists of introand barium orthotitanate, Ba TiO or in two phases of ducing barium titanate powder into a platinum crucible barium titanate and barium dititanate, BaTi O A small and covering the powder with potassium fluoride, which amount of titanium dioxide is known to dissolve in the is a solvent for barium titanate at an elevated temperastoichiometric barium titanate to form a solid solution ture. The barium titanate powder and the flux in the up to approximately 1 molar percent. The excess barium crucible are then heated to a temperature in the range of oxide does not dissolve in barium titanate. Firing process from approximately 1100 C. to 1200 C. and soaked at of the mixture of barium titanate with excess barium such temperature for 8 hours. The amount of flux relative oxide or titanium dioxide is necessary to control the yield. to the barium titanate powder is chosen in such manner A mixture of stoichiometric barium titanate powder and as to assure the presence of undissolved barium titanate barium oxide powder is prepared by using a per se conat the end of the soaking period. The crucible and its conventional technique such as ball-milling in such manner tents are then cooled at a rate in the range of 10 to 30 that the molar ratio of barium oxide to titanium dioxide C. per hour. The barium titanate twin crystals grow in in the mixture is 1.25. The mixture subjected to no presize during the cooling procedure. Decantation of the firing does not produce a higher yield than the stoichiomolten flux at a temperature in the range of from 850 C. metric powder but the mixture pre-fired can produce a to 900 C. is necessary for preventing the twin crystals higher yield. Therefore, it is substantially necessary for from deterioration due to solidification of the flux contactincreasing the twin yield that the barium titanate powder ing with them. The decantation temperature should be a with excess barium oxide be fired at 1200 C. to 1450 C. little higher than the eutectic temperature of the barium prior to introducing into the flux. It is not presently certitanate-potassium fluoridesystem. The crystals left in tain why the composition of the starting powders and the crucible are then cooled slowly to room temperature. prefiring process affect the yield of twin crystals. Yields An inherent disadvantage of the Remeika process is of the butterfly twin are given in Table I in connection the fact that the yield of the barium titanate twin crystals with the said molar ratio.

TABLE I SampleNo 1 2 a 45 07s 9 101112 Molar ratio (Boo/T102) 0.5 0.67 0.83 0. 1.0 1.05 1.1 1.2 1. 25 1.5 1.75 2.0 Yield of twin crystals 10 20 180 130 120 80 Mean ypote use length (mm) 1.0 1.2 1.5 2.0 4.0 4.0 4.5 6.0 6.0 4.5 4.0 3.5

varies over a wide range. This disadvantage was improved by J. W. Nielsen et al. (Journal of the American Ceramic Society, vol. 45, pages 12-17, Jan. 1962). The Nielsen method is characterized by removing substantially all the barium titanate particles larger than 6 microns prior to their introduction into the flux material such as potassium fluoride and produces a high twin yield when the average particle size is approximately 1 micron with a rather sharp The data of Table I are obtained manner:

The sample powders are prepared by air-firing of the mixtures of barium carbonate and titanium dioxide in a desired molar ratio at 1300 C. for 1 hour. Thirty grams of the powder are placed in the bottom of a platinum crucible, 5 cm. in diameter and 5 cm. in height. Seventy grams of potassium fluoride are added to the crucible and in the following then the crucible covered with a lid is heated to l150 C.

and kept at this temperature for 4 hours. The crucible and the contents are then cooled at a rate of 50 degrees per hour until a temperature of approximately 900 C. is attained. The flux, which is still in a liquid state, is decanted at 900 C. and then the crucible and the contents are allowed to cool to room temperature (about C. to about C.) at a rate of 60 degrees per hour. A small amount of the solidified fiux adhering to the surface of the crystals is removed by washing with water. Then twin yields are determined by counting the twin crystals grown in the crucible and are shown in Table I.

Referring to Table I, the yield of twin crystals clearly decreases with a decrease in the molar ratio, whereas it ranges from 130 to 180 at the molar ratio of 1.0 to 1.25 and decreases to 80 with an increase in the molar raLio above 1.25. It will be understood that the yield of twin crystals is lowered by employing barium titanate with an excess of titanium dioxide and is elevated by employing barium titanate with an excess of barium oxide. The

maximum twin yields (approximately 180) are obtained with the aforesaid molar ratio of 1.20 to 1.25 as indicated in Table I. The barium titanate butterfly crystals always exist in the molar ratio of unity, regardless of the variation in the molar ratio of the starting powders.

Referring to Table I, twin crystals from Samples 8 and 9 are liable to contact with each other because of their large population density and to have an average common edge of approximately 6 mm. The butterfly twins from samples other than Samples 8 and 9 have an average common edge of 4.5 mm. or shorter in spite of their small population density. The excess titanium dioxide in the fired starting powders clearly retards the growing rate, whereas the excess barium oxide in the fired starting powders promotes the growing rate and gives a maximum rate at 1.20 to 1.25 of the molar ratio. It will be apparent from Table I that twin crystals obtained from Samples 1 to 4 having excess titanium dioxide are smaller in size than those from Samples 6 to 12 having excess barium oxide.

A proper population density, i.e. number of crystals per unit area of crucible, is preferred for producing large twin crystals without deterioration due to their contact. It has been found according to the present invention that the population density is readily controlled by employing two kinds of barium titanate powders in different compositions; one has the molar ratio smaller than unity and is characterized by a low yield of twin crystals and the other has the molar ratio higher than unity and is characterized by a high yield of twin crystals. A mixture of these different barium titanate powders in a proper weight percentage can control the butterfly twin population density. As the crystal nuclei are in a smaller number and their growing rate becomes higher, the nuclei can grow into larger crystals during the cooling process. It is desirable to produce large barium titanate twin crystals in view of their application in an electronic device.

The powder having the molar ratio of 1.20 and 1.25 gives a maximum twin yield and the common edge of the crystals grown is' approximately 6 mm. in an average length. Barium titanate twins having a common edge of approximately 10 mm. can be produced in a high yield by employing a population density of one per approximately (0.7 cm.) according to this invention.

Although the barium titanate with an excess of titanium dioxide defined by the molar ratio of 0.67 to 0.80 can produce yield of 40-to 50 as shown in the Table I, the crystals so produced are in a small size due to a low growing rate. Therefore, no large crystal can be obtained by employing only the barium titanate with an excess of titanium dioxide. According to the present invention, larger crystals in a high yield can be produced by employing a combination of above said barium titanate with an excess of titanium dioxide and that with an excess of barium oxide which is characterized by a high yield and a high growing rate of twin crystal.

. 4 operable mixing ratio of the two kinds of powder is 50 to weight percent of barium titanate wherein the molar ratio of barium oxide to titanium dioxide is 0.60 to 0.85, and 20 to 50 weight percent of barium titanate wherein the molar ratio of barium oxide to titanium dioxide is 1.0 to 1.5. Preferable mixing ratio is 60 to 70 weight percent of barium titanate wherein the said molar ratio is 0.65 to 0.75 and 30'to'40 weight percent of barium titanate wherein the said molar ratio is 1.10 to 1.25.

Example Sample No. 8 powder is admixed with Sample No. 2 powder in the desired weight percent. The. crystal growing is carried out in the same way as that described above. Thirty grams of the said mixed powder are charged in the platinum crucible of 5 cm. diameter and 5 cm. height and the mixed powder is covered with 70 grams of potassium fluoride. The crucible covered with a lid is heated in a furnace at a rate of 200 C. per hour and maintained at 1150 C. for 4 hours. The results obtained are given in Table II.

TABLE II Weight Percent Mean Hypot- Sample Yield of enuse Length No. Sample Sample Crystals '(mm.)

No. 8 No. 2

Samples No. 8 and No. 2 are the same as in Table I.

The crucible and the content are cooled at a rate of 50 C. per hour until it reaches at approximately 900 C. At this temperature, the flux, which is still in the liquid state, is poured off and the power supply to the furnace is cut off. The crucible and the content are allowed to cool to room temperature. The solidified flux is removed by washing with water. Referring to Table II, 38 and 63 twins having approximately 10 mm. long side are obtained by employing Sample 14 and Sample 15, respectively. The twin crystals are in a size smaller than 10 mm. in length when Sample 13 or Sample 16 is employed.

It should be understood that the examples described above are intended to be illustrative of the present invention. Variations may be made therein within the skill of the art without departing from the spirit and scope of the invention.

What is claimed is:

1. The process of producing butterfly twin crystals of barium titanate in a large size comprising the steps of adding barium titanate powder to a flux material which is a solvent therefor, heating the mixture of barium titanate and flux to an elevated temperature to form a melt, maintaining the said mixture at the said elevated temperature for a period of time during which a portion of the barium titanate dissolves, the ratio of barium titanate to flux in said mixture being greater than the solubility limit of barium titanate in the fiux material at the said elevated temperature, and cooling the melt to promote the crystal growth, the said barium titanate powder comprising (a) 50 to 80 weight percent of barium titanate wherein the molar ratio of barium oxide to titanium dioxide is 0.60 to 0.85 and (b) 20 to 50 weight percent of barium titanate wherein the molar ratio of barium oxide to titanium oxide is 1.0 to 1.5.

6 (b) to weight percent of barium titanate wherein References Cited 2163111510131 ratio of barium oxide to titanium oxide is 1.0 UNITED STATES PATENTS 4. mixture according to claim 3 consisting essen- 2,803,519 8/1957 Karan, 2351 tially of (a) to weight percent of barium titanate 5 2,852,400 9/1958 Rfimelka -39 wherein the molar ratio of barium oxide to titanium di- 2,992,079 7/1961 Lmares et a] X oxide is'O.65 to 0.75 and (b) 30 to 40 weight percent of barium titanate wherein the said molar ratio is 1.10 to HELEN MCCARTHY Primary Exammer' 1.25. W. R. SATTERFIELD, Assistant Examiner. 

1. THE PROCESS OF PRODUCING BUTTERFLY TWIN CRYSTALS OF BARIUM TITANATE IN A LARGE SIZE COMPRISING THE STEPS OF ADDING BARIUM TITANATE POWDER TO A FLUX MATERIAL WHICH IS A SOLVENT THEREFOR, HEATING THE MIXTURE OF BARIUM TITANATE AND FLUX TO AN ELEVATED TEMPERATURE TO FORM A MELT, MAINTAINING THE SAID MIXTURE AT THE SAID ELEVATED TEMPERATURE FOR A PERIOD OF TIME DURING WHICH A PORTION OF THE BARIUM TITANATE DISSOLVES, THE RATIO OF BARIUM TITANATE TO FLUX IN SAID MIXTURE BEING GREATER THAN THE SOLUBILITY LIMIT OF BARIUM TITANATE IN THE FLUX MATERIAL AT THE SAID ELEVATED TEMPERATURE, AND COOLING THE MELT TO PROMOTE THE CRYSTAL GROWTH, THE SAID BARIUM TITANATE POWDER COMPRISING (A) 50 TO 80 WIGHT PERCENT OF BARIUM TITANATE WHEREIN THE MOLAR RATIO OF BARIUM OXIDE TO TITANIUM DIOXIDE IS 0.60 TO 0.85 AND (B) 20 TO 50 WEIGHT PERCENT OF BARIUM TITANATE WHEREIN THE MOLAR RATIO OF BARIUM OXIDE TO TITANIUM OXIDE IS 1.0 TO 1.5. 